The technology of electrophotography is commercially well established. A wide variety of processes and apparatus are used, although they have many characteristics in common. One of the more common forms of this technology involves the use of a plate having a photoconductive insulating layer, generally coated on a conductive layer. Imaging is effected by first uniformly electrostatically charging the surface of the photoconductive layer and then exposing the charged layer to an image or pattern of activating electromagnetic radiation, usually visible light or ultraviolet radiation. This exposure selectively enables the charge in the irradiated areas of the photoconductive insulator to dissipate. The charge which remains in the non-irradiated areas forms a latent image which may be further processed to form a more permanent record of the exposing image or pattern. The most common form of additional processing involves the attraction of particles of material selectively to the charged areas and fusing them to the photoconductive layer or transferring the particles in their imagewise distribution to another surface to which they are more permanently bound by an adhesive or by fusion of the particles themselves. A common electrophotographic construction comprises, in sequence, a substrate, a conductive layer, and photoconductive insulator.
Typical classes of photoconductive materials useful in electrophotography include (1) inorganic crystalline photoconductors such as cadmium sulfide, cadmium sulfoselenide, cadmium selenide, zinc sulfide, zinc oxide, and mixtures thereof, (2) inorganic photoconductive glasses such as amorphous selenium, selenium alloys, and selenium-arsenic, and (3) organic photoconductors such as phthalocyanine pigments and polyvinyl carbazole, with or without binders and additives which extend their range of spectral sensitivity. These systems are well known in the art. For example, U.S. Pat. No. 3,877,935 discusses various problems associated with the crystalline and amorphous classes of photoconductors and shows the use of polynuclear quinone pigments in a binder as a photoconductive layer. U.S. Pat. No. 3,824,099 shows the use of squaric acid methine sensitizing dyes and triaryl pyrazoline charge transport materials as an electrophotographic construction. Cadmium sulfoselenide plates are shown in U.S. Pat. No. 3,764,315, and one of the original disclosures of the use of poly-N-vinylcarbazole as a photoconductive insulating layer is provided in U.S. Pat. No. 3,037,861. A number of diverse organic photoconductors have been disclosed since the development of the carbazole class of photoconductors such as quinones and anthrones (e.g., Hayashi et al., Bull. Chem. Soc. Japan, vol. 39, (1966) pp. 1670-1673), but the carbazoles have continued to attract the greatest attention.
Problems particularly associated with the use of carbazoles as a positive charge transporting material which is capable of supporting the injection of photoexcited holes from a photoconductive layer and is capable of transporting the injected holes also exist in this area of technology. The carbazole condensates with aldehydes as shown in U.S. Pat. No. 4,025,341 have a tendency to oligomerize. This oligomerization can cause a number of problems. The oligomers formed are not of a uniform molecular weight and carbazole content. This creates problems in purification and can create undesirable variations in photoconductive or charge transport properties. Triaryl methanes including a carbazole moiety (as shown in Xerox Disclosure Journal, Vol 3, No. 1, Jan./Feb. 1978, page 7) also tend to be sensitive to oxidation which converts them to an ionic species which will not act as a photoconductive insulator.
Japanese Patent Publication No. 52-34735 discloses carbazole organic photoconductor materials which may have substituents thereon which would inherently prevent oligomerization of the carbazoles. This is not recognized in the disclosure and the carbazoles would still be subject to oxidation problems.